A laser device generally produces a beam of coherent light that has a wavefront of relatively small cross-section. In spite of the small cross-section and the coherency of the beam, the wavefront of a laser typically has a nonuniform power distribution that is stronger in the center than at the outer edges. The power variation may be between five and ten percent Furthermore, to make use of the beam, it is often necessary to expand the cross-sectional area of the beam, thereby spreading the non-uniformity across a larger wavefront.
When conventional lenses are used to expand the beam, the non-uniform power distribution of the wavefront is carried through to the expanded beam. In addition, the non-uniformity of the beam becomes more apparent as the wavefront is now expanded over a greater cross-sectional area. This non-uniformity is often detrimental to the performance of a system utilizing the beam as the system must be designed for some average level of beam power or another approach would be to somehow strip the beam of its lesser power outer portions, possibly through the use of an aperture. Neither of these alternatives enable optimum use of the beam's power and it is very difficult to achieve a uniform power distribution, such as the plus or minus one percent variation that is often desired, by way of conventional lens systems.
Holographic elements have been created to function as conventional bulk optical elements. In these cases, the holographic element, whose orientations and spatial periods are correct for the purpose of diffracting the incident wavefront into a desired output location pattern, shape or image. However, when built to function as a basic lens, these holographic elements would also carry the nonuniform power distribution through to the output pattern, shape or image, thereby also inefficiently using the power of the optical source.
The problem of how to compensate for wavefronts having a nonuniform power distribution is addressed U.S. Pat. No. 4,547,037. In this patent discloses a multi-faceted holographic element which redistributes the light energy of an incident beam onto a second plane disclosed. This is accomplished by constructing each facet as an individual hologram or diffraction grating. Each facet is sized to be inversely proportional to the intensity of the portion of the beam incident thereupon to assuring that substantially the same amount of power passes through each facet. The light transmitted through each facet is diffracted to arrive at different locations on a second plane, relative to their locations in the holographic element. Each of the subholograms or diffraction gratings either expand or contract the portion of the incident beam passing therethrough to illuminate equal, but different, areas on the second plane, thereby producing an output beam at the second plane with a wavefront of nearly constant intensity.
A problem with devices incorporating the teachings of the '037 patent is that if the power distribution of the incident beam upon the surface of the hologram deviates from the design parameters, then the power distribution of the output beam at the second plane will be similarly affected and thus no longer uniform. In optical systems, there are many causes for such deviation in the power distribution of the incident beam could occur. For example, power fluctuations due to the age of the components, or simply the replacement of the source due to failure. In addition, any misalignment within the system due to shock or age will produce an output wavefront having a non-uniform power distribution.
What is needed is an relatively inexpensive way to convert an incident optical beam having a wavefront with a non-uniform spatial energy distribution to an output beam having a substantially uniform spatial energy distribution that is relatively insensitive to fluctuations in positioning of the incident beam and spatial energy distributions within the incident beam.
Further, what is needed is a relatively inexpensive way to convert an incident optical beam having a wavefront with a non-uniform spatial energy distribution to an output beam having a preselected spatial energy distribution using a hologram that does not have regular breaks between facets in order to better minimize the intensity variations on the output plane caused by regular breaks between facets.
Further, what is needed is a relatively inexpensive way to convert an incident optical beam having an arbitrary wavefront to an output beam having preselected attributes, including preselected angular spread, such that the output beam is useful in photolithography. Photolithographic exposure systems are used to image the pattern of a mask onto a wafer for the purposes of exposing resist, or photoresist, on the wafer in a pre-determined pattern. Subsequent processing of the wafer results in the completion of layers that eventually form the desired device, such as an integrated circuit.
When the mask is used in a projection lithography system, such as a laser stepper with a 5:1 or 10:1 reduction ratio, the mask is often referred to as a reticle. The reticle or mask is typically formed by chrome regions on a transparent substrate. The chrome regions of the mask block the incident light, thereby imposing the pattern of the mask as an intensity variation on the light. In a 5× laser stepper, the pattern of the reticle is reduced by a factor of 5 as imaged onto a wafer. Typically, in this application, the beam illuminating the diffractive is relatively uniform and has a rather narrow cone angle of divergence, i.e., limited spatial and angular energy distributions.
While masks and reticles control the intensity of light on the wafer, there is a need for an element that controls the angular distribution of the light on the wafer. By modifying the particular angular distribution of the light illuminating the wafer, one can extend the depth of the field and resolving power of photolithographic exposure systems. This element should ideally be inexpensive and relatively insensitive to fluctuations in positioning of the incident beam and to fluctuations in the spatial energy distributions of the incident beam.
Moreover, what is needed is a relatively inexpensive way to convert a collimated incident optical beam having a wavefront with non-uniform spatial energy distribution to an output beam having a preselected spatial energy distribution, or a preselected beam shape, that is relatively insensitive to fluctuations in positioning of the incident beam and spatial energy distributions within the incident beam. Additionally, what is needed is a relatively inexpensive way to convert an incident optical beam having a wavefront with non-uniform spatial energy distribution to an output beam having preselected attributes, such as spatial energy distribution, or a preselected beam shape, or a preselected angular energy distribution, that is relatively insensitive to fluctuations in positioning of the incident beam and spatial energy distributions within the incident beam.